Subduction of a rifted passive continental margin: the Pohorje case of Eastern Alps\u2013constraints from geochronology and geochemistry

Ruihong Chang,Johann Genser,Qingbin Guan,Qianwen Huang,Sihua Yuan,Yongjiang Liu,Wei Jin,Weimin Li,Franz Neubauer

doi:10.1186/s00015-020-00369-z

Abstract

This study presents geochronological and geochemical data from newly dated Permian granitic orthogneisses associated with the Eclogite-Gneiss unit (EGU) from the southernmost part of the Austroalpine nappe stack, exposed within the Pohorje Mountains (Slovenia). LA-ICP-MS zircon U–Pb ages of two samples of the augen-gneisses are 255 ± 2.2 Ma and 260 ± 0.81 Ma, which are interpreted as the age of magmatic crystallization of zircon. In contrast, all round zircons from leucogneisses give Cretaceous ages (89.3 ± 0.7 Ma and 90.8 ± 1.2 Ma), considered as the age of UHP/HP metamorphism. The round zircons overgrew older euhedral zircons of Permian and rare older ages tentatively indicating that these rocks are of latest Permian age, too. Zircon εHf(t) values of the four orthogneiss samples are between − 13.7 and − 1.7 with an initial 176Hf/177Hf ratio ranging from 0.282201 to 0.282562; TDMC is Proterozoic. The augen-gneisses show geochemical features, e.g. high (La/Lu)N ratios and strong negative Eu anomalies, of an evolved granitic magma derived from continental crust. The leucogneisses are more heterogeneously composed and are granitic to granodioritic in composition and associated with eclogites and ultramafic cumulates of oceanic affinity. We argue that the Permian granitic orthogneisses might be derived from partial melting of lower crust in a rift zone. We consider, therefore, that segment of the EGU is part of the distal Late Permian rift zone, which finally led to the opening of the Meliata Ocean during Middle Triassic times. If true, the new data also imply that the Permian stretched continental crust was potentially not much wider than ca. 100 km, was subducted and then rapidly exhumed during early Late Cretaceous times.

Highlights

A-type subduction is considered to occur at the initial stage of continent–continent collision after consumption of an intervening oceanic lithosphere and subduction of the stretched passive margin
To resolve these alternative scenarios, we investigated the southernmost part of the Eclogite-Gneiss Unit (EGU) and associated units of Cretaceous metamorphic age (Fig. 1a), which is part of the Austroalpine nappe stack and which is exposed in the Pohorje Mountains in the southeasternmost part of the Eastern Alps (Fig. 1b)
This paper presents the first evidence for Permian granites within or associated with Eclogite-Gneiss unit (EGU) exposed in the Pohorje Mountains of Slovenia

Summary

Introduction

A-type subduction is considered to occur at the initial stage of continent–continent collision after consumption of an intervening oceanic lithosphere and subduction of the stretched passive margin. The main tectonic events of the Austroalpine nappe stack, which stretches from the Eastern Alps to the Western Carpathians, are (1) rifting and lithospheric thinning in the Permian following the Variscan orogeny and deposition of Late Carboniferous Variscan molasse; (2) Middle Triassic opening of the oceanic Meliata basin; (3) Jurassic convergence and subduction of the Meliata basin; (4) internal nappe stacking within the Austroalpine nappe complex after closure of the Meliata oceanic basin during Early Cretaceous times; and (5) Paleogene collision of the southern margin of the stable European continent and the Austroalpine nappe complex after the subduction of the South Penninic ocean under the Austroalpine microplate (Neubauer et al 2000; Schuster et al 2008; Froitzheim et al 2008; Thöni et al 2008; Janák et al 2004, 2006, 2009; Sassi et al 2004; Miller et al 2005a, 2005b, 2007; Bruand et al 2010; Kirst et al 2010)

Methods

Results

Conclusion